The present invention relates to a precise indexing apparatus and method for accurately shifting or moving sheet material over the work surface of a table. More particularly, the present invention is concerned with an indexing apparatus that can be attached and detached from a tool or instrument carriage between work operations.
U.S. Pat. No. 3,844,461 entitled "Precise Indexing Apparatus and Method" filed by H. J. Gerber and S. C. Robison discloses an apparatus that is used to shift sections of sheet material over the work surface of a plotting table in a numerically controlled plotting machine. A strip of plotting material having a length much greater than the dimensions of the work surface of the plotting table receives plotting information along its entire length by first plotting on one section of material positioned on the work surface, then shifting a new section of material onto the surface and continuing the plotting operation. The shifting and plotting operations may be repeated as often as desired or needed to complete a plot.
A significant advantage of the prior art apparatus in U.S. Pat. No. 3,844,461 is that the indexing of the sheet material is performed by the carriage which ordinarily carries the plotting instrument or tool that works on the sheet material. Thus the sheet material in indexed or moved with the same degree of accuracy as the instrument. Furthermore, by clamping the sheet material to the instrument carriage for indexing, the sheet material and the instrument have the same positional relationship before and after the indexing operation. Thus any fixed errors such as warpage of the ways or guides which hold the instrument carriage will not destroy the positional reference of the instrument and sheet material. Accordingly, continuity between the plot made prior to the indexing operation and the plot made after the indexing operation is maintained. For a more detailed explanation of the indexing operation and the utility of the apparatus reference is made to U.S. Pat. No. 3,844,461. In copending application Ser. No. 797,000 filed May 16, 1977, now U.S. Pat. No. 4,091,980, by H. J. Gerber and entitled "Apparatus for Advancing Sheet Material", a similar apparatus is disclosed.
In the prior art devices discussed above, couplings are fastened to the instrument carriage and move with the carriage during plotting or other work operations. Magnetic couplers such as shown in FIG. 10 of the referenced patent or the elongated bar in FIGS. 11 and 12 contribute to the inertia of a light-weight carriage and, correspondingly, depreciate the overall performance of the plotting apparatus. Furthermore, a friction coupler such as shown in the above-referenced application Ser. No. 797,000 cannot be utilized in systems where the work surface of the plotting table has a relatively high coefficient of friction or in instances where, for example, a sheet of carbon paper is positioned under the plotting material to generate duplicate copies of the plot or simply to improve the reproducibility of the plot.
Accordingly, it is a general object of the present invention to provide an indexing apparatus which constitutes an improvement by allowing light-weight carriages to operate at maximum performance and allows sheet material to be precisely indexed over work surfaces having high coefficients of friction or carbon paper coverings.